The present invention relates to a fluid pressure control device for an automatic transmission, and in particular to a fluid pressure control device for an automatic transmission which controls the shift timing upon power-off up-shifting.
In general, when an automatic transmission is not provided with a one-way clutch at each of a frictional engaging portion of a low speed side and a frictional engaging portion of a high speed side, a timing valve is disposed in a fluid pressure control circuit so as to establish a well-timed shift operation through the rapid discharge of fluid pressure from the frictional engaging portion of the low speed side upon detection of an increase in the fluid pressure in the frictional engaging portion of the high speed side.
In FIG. 5, there is illustrated a conventional timing valve 1 which establishes the foregoing operation in the automatic transmission and which is set to establish engaging operations based on a table shown in FIG. 8. That is to say, the timing valve 1 is set to establish a well-timed drain of fluid pressure from a second brake B1 which is regarded as the frictional engaging portion of the low speed side upon an upward shifting from the second gear position to the third gear position.
The timing valve 1 has an input port 1a for receiving a pressure signal Ps from a pressure source via a throttle valve which indicates an opening of the throttle valve. Another input port 1b is provided on the timing valve 1 in order to receive a direct clutch fluid pressure Pc2 for engaging a direct clutch C2 as the frictional engaging portion of the high speed side as a result of an upward shifting toward a third gear side or position of a 2-3 shift valve 2 and the resulting fluid pressure or the supplied direct clutch fluid pressure Pc2 is set to oppose a biasing force of a spring 1B across a spool 1A. In addition, a third input port 1c is provided on the timing valve 1 so as to be inputted with a second brake fluid pressure PB1. If the second brake fluid pressure PB1 is drained from the timing valve 1 through a drain port 1d and an orifice 3 which results in the downward movement of the spool IA against the biasing force of the spring 1B, the drain port 1d is closed which leads to a drain of the second brake fluid pressure PB1 from the timing valve 1 through another drain port 1e.
Thus, when the 2-3 shift valve 2 is switched from the condition for the second gear stage to the condition for the third gear stage, as shown in FIG. 6(a), the second brake fluid pressure PB is decreased gradually due to its drain through the orifice 3 and contrary to this the direct clutch fluid pressure Pc2 is increased. As soon as the the direct clutch fluid pressure Pc2 becomes or reaches a set value or a timing valve switching pressure PT1, the spool 1A is then brought into downward movement against the biasing force of the spring 1B, a rapid drain of the second brake fluid pressure PB1 from the timing valve 1 through the drain port 1e is established. The foregoing timing control wherein the disengagement of the second brake B1 and the engagement of the direct clutch C2 are established concurrently assures the prevention of shock upon shifting.
Furthermore, as shown in FIG. 4, when a power-off up-shifting is established along a direction of "X" under a change of the throttle valve from 100% opening to 5% opening for example, the pressure signal Ps which depends on the opening of the throttle valve is supplied to the input port 1a, and the drain of the fluid pressure from the second brake B1 is established which is more rapid than normal. Thereafter, an underlap control is performed wherein the direct clutch C2 is engaged gradually in order to establish a smooth shifting.
However, in the foregoing or conventional fluid pressure control device which is adopted in the automatic transmission, the underlap operation is initiated only when the opening of the throttle valve is less than a set value of 10% for example, even though the pressure signal Ps is supplied to the input port 1a. This means that even though the power-off up-shift is established while the the opening of the throttle valve Th is greater than the foregoing degree, the underlap operation cannot be established, thereby performing the disengagement of the second brake B1 and the engagement of the direct clutch in a timing which is not less than that in the power-on up-shifting. Thus, as shown in FIG. 6(b), the torque variation of the output shaft becomes large, which leads to an excessive or large shock upon shifting.
In addition, in the foregoing structure, the underlap control is set to be established based on only the opening of the throttle valve, which brings an unexpected underlap control when the releasing speed of the accelerator is small.